WASHINGTON (CNN) --Longtime consumer advocate Ralph Nader entered the Democratic presidential race as an independent candidate Sunday, marking the third time he has made a run for the White House.

This time he is running as an independent, so the process is more formidable and more complicated. Here's how:

In 2000, Nader's name was on the ballot in 43 states and the District of Columbia.

But then he was running as a candidate of the Green Party, which already qualified for a ballot slot in many states based on its showing in past elections. And the Greens had an established organization to collect signatures to get Nader on the ballot in other states.

Nader's Web site says it needs to collect 1.5 million registered voters signatures in just a few months. On top of that, election laws vary greatly from state to state and can be confusing.

Forty-eight states and the District of Columbia require independent presidential candidates to collect petition signatures to appear on the ballot. Each state requires a different number of signatures. Colorado and Louisiana require a fee but no signature gathering to appear on the ballot.

"There's a tremendous bias in state laws against third parties and independent candidates, bred by the two major parties, who passed these laws. They don't like competition. So it's like climbing a cliff with a slippery rope," Nader said Monday on NBC's "Meet the Press."

The five states with the highest per capita number of signatures required to qualify for the ballot are Georgia, Indiana, Oklahoma, North Carolina and Texas, according to Richard Winger, editor of Ballot Access News, a monthly newsletter on state ballot access laws.

Nader's first deadline is May 13 in Texas. To get on the ballot in the president's home state, Nader needs to collect about 64,000 valid signatures from people who did not vote in either major party primary.

In California, Nader needs to collect more than 150,000 signatures.

In some states, Nader needs to collect as few as 1,000 valid signatures. In two states, all he has to do is file a notice of intent to run and pay a fee.

Most states allow write-in candidates for president but five states -- Hawaii, Louisiana, Nevada, Oklahoma, and South Dakota -- ban them.

North Carolina allows write-in candidates, but they must register as write-in candidates in July. In 2000, write-in votes for Nader were not counted because he did not file by the state's deadline.

All states officially recognize the Democratic and Republican parties. Some states also recognize minor parties and allow those parties' candidates to appear automatically on the ballot without collecting signatures.

Past independent candidates, such as Ross Perot in 1992, have aligned themselves with different parties in different states to more easily gain ballot access.